This invention relates to a method and device for the production of hydrogen by water splitting.
Global environmental concerns have ignited research to develop energy generation technologies which leave minimal ecological damage concerns of global climate change are driving nations to develop electric power generation technologies and transportation technologies which reduce carbon dioxide emissions. Hydrogen is considered the fuel of choice for both the electric power and transportation industries.
The need to generate ever larger amounts of hydrogen is clear. Outside of direct coal liquefaction, other major industrial activities, such as petroleum refining, also require hydrogen. Collectively, petroleum refining and the production of ammonia and methanol consume approximately 95 percent of all deliberately manufactured hydrogen in the United States. As crude oil quality deteriorates, and as more stringent restrictions on sulfur, nitrogen and aromatics are imposed, the need for more hydrogen for the refining industry will increase.
Hydrogen production, as a consequence of other processes, is significant. A number of industries requiring hydrogen produce effluents containing significant amounts of unused hydrogen. However, this hydrogen requires clean-up prior to re-use. Furthermore, hydrogen is produced from the combustion of oil, methane, coal, and other petroleum-based materials. However, this hydrogen must be separated from other combustion gases, namely carbon dioxide, in order to be of use.
Petroleum refineries currently use cryogenics, pressure swing adsorption (PSA), and membrane systems for hydrogen recovery. However, each of these technologies have their limitations. For example, because of its high costs, cryogenics generally can be used only in large-scale facilities which can accommodate liquid hydrocarbon recovery.
Membrane-based PSA systems require large pressure differentials across membranes during hydrogen diffusion. This calls for initial compression of the feed prior to contact to the upstream side of polymeric membranes and recompression of the permeate to facilitate final purification steps. Not only are these compression steps expensive, but PSA recovers less feedstream hydrogen and is limited to modest temperatures. U.S. Pat. No. 5,447,559 to Rao discloses a multi-phase (i.e. heterogeneous) membrane system used in conjunction with PSA sweep gases.
Many membrane systems have been developed in efforts to efficiently extract target material from feed streams. Some of these membrane systems (U.S. Pat. Nos. 5,030,661, 5,645,626, and 5,725,633) are synthetic based and incorporate polyimides and polyethersulphones. Such organic membranes also have limited temperature tolerance.
Proton-exchange membranes have high proton conductivities, and as such, are currently in development for fuel-cell applications and hydrogen pumps. One such application is disclosed in U.S. Pat. No. 5,094,927 issued to Baucke on Mar. 10, 1992. However, inasmuch as these membranes have relatively low electronic conductivities, they are not viable for hydrogen recovery scenarios, primarily because these membranes require the application of an electric potential to drive proton transport.
Water disassociates into oxygen and hydrogen at high temperatures, and the disassociation increases with increasing temperature:             H      2        ⁢          O      ⁡              (        g        )              ⇔            H      2        +                  1        2            ⁢                        O          2                .            
Because of the small equilibrium constant of this reaction, the concentrations of generated hydrogen and oxygen are very low even at relatively high temperatures i.e., 0.1 and 0.042% for hydrogen and oxygen, respectively at 1600xc2x0 C. However, significant amounts of hydrogen or oxygen could be generated at moderate temperatures if the equilibrium were shifted toward disassociation. While hydrogen can also be produced by high-temperature steam electrolysis, the use of a variety of membranes including mixed-conducting membranes offers the advantage of requiring no electric power or electrical circuitry. In considering the above disassociation equation, it appears at first blush that the removal of either hydrogen or oxygen would continue to drive the reaction toward disassociation. However, that is not the entire case as will be hereinafter set forth.
An object of the invention is to provide a device and method for splitting water into its component parts wherein the driving force of the reaction remains relatively high.
Another object of the invention is to provide a device and method for disassociating water into oxygen and hydrogen using substantially gas impervious solid electron-conducting membranes selectively removing the components of the disassociation reaction.
Yet another object of the present invention is to provide a device and method for separating water into hydrogen and oxygen in which membranes are used which selectively pass atomic hydrogen or protons on the one hand, and selectively pass atomic oxygen or oxide ions on the other hand.
Yet another object of the present invention is to provide a device and method of water splitting in which either single or two-phase membranes are used selectively to separate hydrogen and oxygen after disassociation.
The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.